To overcome the problem of active device switching losses in power converters, while enabling operation at higher switching frequencies, soft-switching converters have been developed. For example, D. M. Divan describes an active-clamped resonant dc link (ACRDCL) converter in U.S. Pat. No. 4,864,483, issued Sept. 5, 1989, which is incorporated by reference herein. In the ACRDCL, a resonant circuit, comprising a resonant inductance and a resonant capacitance, is connected to a dc power supply and to a resonant dc bus supplying an inverter. Stable high-frequency oscillations of the resonant circuit provide a unidirectional voltage across the resonant dc bus which reaches zero voltage during each resonant cycle. The inverter switching devices are controlled to switch on and off only at times when the dc bus voltage is zero, thereby substantially avoiding switching losses in the inverter. Despite its advantages, the ACRDCL converter has the following disadvantages: (1) The clamp switch is required to operate at the resonant frequency of the resonant circuit, which can lead to significant switching losses; (2) Inverter switches are subject to high voltage stresses above the dc power supply voltage; and (3) The converter must maintain continuous resonant operation on the dc link.
An alternative soft-switching converter is an auxiliary resonant commutated pole (ARCP) converter which employs an auxiliary resonant snubber for each inverter phase, which snubber is triggered by auxiliary switching devices to achieve zero-voltage switching in the inverter main devices. According to the ARCP control described in commonly assigned U.S. Pat. No. 5,047,913 of R. W. De Doncker and J. P. Lyons, issued Sept. 10, 1991 and incorporated by reference herein, the auxiliary resonant commutation circuit is triggered into conduction by gating one of the auxiliary power switches which connect the auxiliary resonant circuit to a forcing potential of half the dc link voltage. The auxiliary resonant circuit produces a sinusoidal half-cycle of current which results in the resonant bus voltage swinging between the positive and negative rails of the dc link. After this half cycle is completed, the auxiliary device turns off. Thus, the auxiliary switches are required to conduct only when the main devices are in the process of switching, and they naturally turn off at zero current as the resonant cycle seeks to reverse direction. Advantageously, in contrast to the ACRDCL converter described hereinabove, the ACRP converter is not required to resonate continuously. However, the ARCP circuit requires auxiliary devices and resonant inductors for each phase leg of the inverter. Furthermore, in converters for switched reluctance machines (SRM's), the number of auxiliary resonant commutation circuits required can even be twice the number of motor phases because the SRM inverter has no common connection point for the upper and lower switching devices in a given phase leg.
Therefore, it is desirable to provide a quasi-resonant soft-switching converter that has only one auxiliary resonant commutation circuit and is required to provide a soft-switching opportunity only when the inverter switches to another state. To be practical, such a quasi-resonant converter should exhibit low current and voltage stresses and should use a minimal number of circuit components. Moreover, such a quasi-resonant dc link converter should be capable of producing quasi-PWM waveforms, i.e., pulse width modulated waveforms with a minimum dead-time equal to one-half the resonant cycle, thereby eliminating the subharmonics that are otherwise created by the discrete pulse problems associated with the discrete pulse modulation (DPM) scheme of the ACRDCL converter.